Cellular mobile radiotelephones allow users in cars to remain in contact with the landline telephone system while traveling. This form of communication typically has the disadvantage of requiring the user to hold a telephone handset to communicate. This can cause problems when another task must be performed simultaneously with communicating, such as driving or writing.
This problem has been solved by the implementation of a hands-free mode of operation for the cellular radiotelephone. This is accomplished by using a speaker and microphone separate from the speaker and microphone in the radiotelephone's handset. The extra speaker can be built in to the radiotelephone housing or in a separate housing. The hands-free microphone, for improved voice reception, is typically separate from the radiotelephone housing and is mounted near the user, such as on the driver's visor.
While the external speaker and microphone free the user's hands during a call, the user must still look down at the radiotelephone to operate the controls for dialing a number. If the user is driving at the time a call is made, a dangerous situation can result because of the time required to look down to dial a number.
One solution to this problem is to locate a limited number of control buttons in the hands-free microphone housing. If the microphone is mounted near the user, the need to look away from the road is reduced and, if a relatively small number of buttons are used, the user may even be able to use the buttons by memorizing their locations.
The mounting of controls in the microphone housing requires a greater number of wires in the interface cable to the radiotelephone than if only the microphone were present. A larger number of wires in the cable will result in a thicker cable, creating a problem of installing the cable between the radiotelephone and the hands-free microphone without detracting from the automobile interior's appearance. There is a resulting need, therefore, to provide control functions in the hands-free microphone using a minimum number of wires.